For time differences measurement the “Tick Count (ms)” VI from the standard LabVIEW palette can be used.
Note, that SDK version 4.2.1 has a bug fix in this function.
The tick count starts when the application starts and overflows after 2^32ms = 49,7 days.

If you want higher resolution than 1ms you can use the following code:

drag and drop this VI snippet into a LabVIEW block diagram.